LED drivers control a variety of functional aspects of an LED including the flow of current through a LED or LED array. Many LED drivers control the flow of current using some form of a constant current control circuit.
FIG. 1 illustrates a number of square wave pulses associated with an implementation of a constant current control mechanism in an LED circuit for dimming one or more LEDs. The pulses are controlled by an active control element that is gated. A dimming signal 102 provides a desired or ideal effective value of the current. Dimming signal 102 tells the current controller how to control the “temporal density” of the LED current by providing a temporal density function to the controller. A temporal density function is used to modulate the density of current pulses over time by setting/defining current pulse parameters such as, the number, position, timing and/or repetition of pulses in a given time interval. Thus, the current flows in intervals and is ON for a portion of time and OFF for a portion of time during a particular interval according to the temporal density function. This produces a dimming effect upon the LED(s). In other words, the light output of the LED is broken up into pulses by the temporal density function. By controlling the ratio of the time this temporal density function is high (or “ON”) to the time it is low (“OFF) the average output of the LED is controlled. Since the eye has a rather long time constant, with a proper selection of the frequency range for the components of the temporal density function, the eye perceives this light output to interpret an intensity level.
Errors introduced by the current control mechanisms of the LED circuit are not well controlled. For instance, errors occur when the current goes from an OFF state to an ON state because there is a ramp up time, corresponding to time interval 108, where the actual current 106 is less than the ideal current 104. Likewise, when the current goes from an ON state to an OFF state there is a ramp down time 110 where the actual current 106 exceeds the ideal current 104. The error associated with the ramping up time does not necessarily cancel out the error associated with the ramping down time. Thus, these errors produce an actual average current 114 that is different than the ideal average current 116 set by the dimming signal 102. These and other errors in the LED circuit cause distortions in the color and/or intensity of light emitted by the LED(s).